1. Field of the Invention
The present invention relates to a display device, and more particularly, to an electric field driven liquid crystal lens in which the position of a spacer is adjustable to prevent a lens surface error, and a stereoscopic display device using the same.
2. Discussion of the Related Art
At present, services for rapid dissemination of information, based on the construction high-speed information communication networks, have developed from a simple “listening and speaking” service, such as current telephones, to a “watching and listening” multimedia type service based on digital terminals used for high-speed processing of characters, voices, and images. Ultimately, such services are expected to be developed into hyperspace 3-dimensional (3D) stereoscopic information communication services enabling virtual reality and stereoscopic viewing free from the restrains of time and space.
In general, stereoscopic images representing 3-dimensions are realized based on the principle of stereo-vision via the viewer's eyes. However, since the viewer's eyes are spaced apart from each other by about 65 mm, i.e. a binocular parallax, the left and right eyes perceive slightly different images due to a positional difference between the two eyes. Such an image difference due to the positional difference between the two eyes is called binocular disparity. A 3D stereoscopic image display device is designed based on binocular disparity, allowing the left eye to view only an image for the left eye and the right eye to view only an image for the right eye.
Specifically, the left and right eyes view different 2-dimensional (2D) images, respectively. If the two different images are transmitted to the brain through the retina, the brain accurately combines the images, reproducing depth perception and realism of an original 3D image. This ability is referred to as stereoscopy (stereography), and a display device to which stereoscopy is applied is referred to as a stereoscopic display device.
In the meantime, stereoscopic display devices may be classified based on constituent elements of a lens which realizes 3D images. In one example, a lens using a liquid crystal layer is referred to as an electric field driven liquid crystal lens.
Generally, a liquid crystal display device includes two electrodes opposite each other, and a liquid crystal layer interposed between the two electrodes. Liquid crystal molecules of the liquid crystal layer are driven by an electric field created when voltages are applied to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy characteristics. Here, polarization refers to a change in molecular alignment according an electric field, which is caused as electrons in liquid crystal molecules are gathered to opposite sides of the liquid crystal molecules when the liquid crystal molecules are under the influence of an electric field. Also, optical anisotropy refers to a change in path or polarization of light to be emitted according to an incidence direction or polarization of incident light, which is caused by an elongated shape of liquid crystal molecules and the above-mentioned molecular arrangement direction.
Accordingly, the liquid crystal layer has a transmittance difference due to voltages applied to the two electrodes, and is able to display an image by varying the transmittance difference on a per pixel basis.
Recently, an electric field driven liquid crystal lens in which a liquid crystal layer serves as a lens based on the above-described characteristics of liquid crystal molecules has been proposed.
Specifically, a lens is designed to control a path of incident light on a per position basis using a difference between a refractive index of a lens constituent material and a refractive index of air. In the electric field driven liquid crystal lens, if different voltages are applied to electrodes located at different positions of the liquid crystal layer so as to create an electric field required to drive the liquid crystal layer, incident light introduced into the liquid crystal layer undergoes different phase variations on a per position basis, and as a result, the liquid crystal layer is able to control the path of the incident light in the same manner as an actual lens.
An electric field driven liquid crystal lens according to the related art will be now described with reference to the accompanying drawings. FIG. 1 is a sectional view illustrating the electric field driven liquid crystal lens according to the related art, and FIG. 2 is a schematic view illustrating a conformation of the electric field driven liquid crystal lens of FIG. 1.
As illustrated in FIG. 1, the electric field driven liquid crystal lens according to the related art includes first and second substrates 10 and 20 opposite each other, and a liquid crystal layer 30 formed between the first substrate 10 and the second substrate 20. First electrodes 11 are arranged on the first substrate 10 and are spaced apart from one another by a first distance. In the two neighboring first electrodes 11, a distance from the center of one first electrode 11 to the center of the other first electrode 11 is referred to as a “pitch”. Repeating the same pitch for the respective first electrodes results in a pattern. A second electrode 21 is formed over the entire surface of the second substrate 20 opposite the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metal. The liquid crystal layer 30 is formed in a space between the first electrodes 11 and the second electrode 21. Liquid crystal molecules of the liquid crystal layer 30 have a parabolic potential surface due to a property reacting according to the intensity and distribution of an electric field and thus, have a phase distribution similar to that of the electric field driven liquid crystal lens as illustrated in FIG. 2.
In addition, ball spacers 40 are distributed to support a gap between the first substrate 10 and the second substrate 20. These ball spacers 40 are randomly dispersed on any one of the first substrate 10 and the second substrate 20 and thus, have mobility on the substrate rather than being fixed at specific positions.
The above-described electric field driven liquid crystal lens is realized under the condition that high voltages are applied to the first electrode 11 and the second electrode 21 is grounded. With this voltage condition, a vertical electric field is strongest at the center of the first electrode 11, and the intensity of the vertical electric field decreases away from the first electrode 11. Accordingly, if the liquid crystal molecules of the liquid crystal layer 30 have positive dielectric anisotropy, the liquid crystal molecules are arranged according to the electric field in such a way that the liquid crystal molecules are upright at the center of the first electrode 11 and are gradually tilted approximately horizontally away from the first electrode 11. As a result, in view of light transmission, an optical path is shortened at the center of the first electrode 11, and is lengthened with increasing distance from the first electrode 11, as illustrated in FIG. 2. Representing the length variation of the optical path using a phase surface, the electric field driven liquid crystal lens has light transmission effects similar to a lens having a parabolic surface. Here, the second electrode 21 causes behavior of the electric field created by the liquid crystal molecules, making a refractive index of light spatially take the form of a parabolic function. The first electrode 11 corresponds to a lens edge region.
In this case, relatively high voltages are applied to the first electrodes 11 rather than the second electrode 21. Therefore, as illustrated in FIG. 2, an electric potential difference occurs between the first electrodes 11 and the second electrode 21. In particular, a steep lateral electric field is created around the first electrodes 11. Accordingly, liquid crystals have a slightly distorted distribution rather than a predetermined distribution, whereby a refractive index of light cannot exhibit parabolic spatial distribution, or movement of the liquid crystals is excessively sensitive to voltage variation.
The above-described electric field driven liquid crystal lens according to the related art may be realized, without a lens having a parabolic surface, by arranging electrodes on two substrates with liquid crystals interposed therebetween and applying voltages to the electrodes.
FIGS. 3A and 3B are a plan view and a sectional view, respectively, illustrating a region occupied by a spacer and a region having an effect on transmission of light due to the existence of the spacer.
For example, if it is assumed that a ball spacer 45 is located at a position of an electric field driven liquid crystal lens as illustrated in FIG. 3A, an area defined by tripling the diameter of the ball spacer 45 as illustrated in FIG. 3B may intercept transmission of light directed from the bottom of the electric field driven liquid crystal lens, or light may be refracted at a surface of the ball spacer 45, causing distortion of light to be transmitted.
Distortion in transmission of light occurs in an area of (3r)2π where “r” represents radius of the spacer 45. Actually, this distortion occurs in an area equal to 9 times an area occupied by the ball spacer 45. For example, if it is assumed that the ball spacer 45 occupies 0.5% of the entire substrate area, the above described distortion occurs in an area of 4.5% of the entire substrate area. Therefore, a distorted lens surface may be identified with the naked eye, or an abnormal 3D display region may occur.
The above-described electric field driven liquid crystal lens in the related art has the following problems. First, it is necessary to provide a spacer to maintain an interval between the first substrate and the second substrate in consideration of mobility of the liquid crystals filled between the first substrate and the second substrate. However, the spacer itself cannot function as a lens. Even a region around the spacer may exhibit distortion in a transmission direction of light, or may intercept light due to the existence of the spacer. Second, the distortion in the transmission of light occurs in an area equal to approximately 9 times an area of the spacer, rather than occurring only in an area of the spacer and thus, the entire electric field driven liquid crystal lens may exhibit a distinguishable lens error as the distortion occurs due to light transmitted laterally. Third, the function of the related art electric field driven liquid crystal lens may deteriorate. If lens error occurs, respective lens regions of the electric field driven liquid crystal lens may have different refractive indices and in turn, such a refractive index difference may cause an irregular lens interface.